In-stream Bioreactor for Agricultural Nitrate Treatment
University of Waterloo
Nitrate from agricultural activity contributes to nutrient loading in surface water bodies. This study describes ongoing monitoring of a novel in-stream bioreactor that uses carbonaceous solids (woodchips) to promote denitrification of agricultural drainage. The reactor (40 m3) was trenched into the bottom of an existing agricultural drainage ditch in southern Ontario (Avon site) and a cobble riffle was used to induce flow through the reactor. Monitoring in the second and third years of operation and a comparison of laboratory measured reaction rates for the Avon media and other woodchip bioreactors that have been in operation for up to eight years, suggests that such reactors have the potential to sustain removal rates on the order of 25-250 mg N m-2 hr-1 over decadal timeframes. These removal rates substantially exceed those reported for constructed wetlands. Stream channel narrowing using cobble rip-rap eliminated a siltation problem experienced during the first year of operation. Recent monitoring shows that such in-stream bioreactors can generate greenhouse gases, N2O and CH4, at rates similar to other N-rich agricultural and aqueous environments and can generate methyl mercury at rates similar to other wetland environments. However, production of these potentially undesirable secondary constituents can be reduced or eliminated by control of the redox condition within the reactor. This is easily done at the Avon site using an adjustable height outlet pipe which allows flow through the reactor to be controlled. In-stream bioreactors are simple to install and maintain, they can utilize existing agricultural drainage networks and they could be suitable for larger scale applications. They could also work well in combination with other nitrate control methods such as the use of constructed wetlands.